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Weather modification, commonly called cloud seeding, is the practice through which selected clouds are treated with a well-timed placement of a specific material to accelerate the development of precipitation. The characteristics of the clouds themselves determine whether cloud seeding can be effective. Cloud seeding works by helping to correct existing deficiencies in precipitation processes.

Cloud seeding is most commonly used to increase precipitation, both in warm and cold seasons (summer and winter). Spring and summer operations are usually conducted from aircraft, as the clouds of interest are convective (cumuliform). Winter storms are most often seeded to increase the snowfall over mountainous terrain. In these cold-season programs, seeding can be done either from aircraft or from ground-based facilities.

Warm season cloud seeding is also used to mitigate hail damage to homes, automobiles, property and crops. Hail damage mitigation efforts utilize airborne delivery of the seeding material.

In addition, cold fogs, those made of cloud droplets but existing at cold temperatures (less than 0°C); can often be dissipated by seeding with ice-forming agents.

Warm cloud seeding stimulates precipitation production in clouds that are not cold enough to produce ice. Hygroscopic materials are introduced to encourage the formation of larger cloud droplets, which leads to the development of rain through the collision and coalescence process. Cold cloud seeding works by stimulating ice formation in clouds cold enough to develop it. This accelerates the growth of precipitation-sized particles.

The success of cloud seeding programs for increased participation can be measured using precipitation gauge data to compare rainfall or snowfall during comparable seeded and non-seeded periods. Another evaluation technique relies on the comparison of seeded seasons with other non-seeded seasons in the same location. For warm season seeding projects, weather radar is sometimes used for evaluations, because all the clouds within range are measured. Some wintertime projects, designed to increase snowfall and subsequent runoff, measure changes in stream flows to assess the impact of seeding efforts.

Seeding clouds does not remove enough moisture from the atmosphere to affect precipitation downwind.

This is one of our most common questions, and to fully understand this answer, one must look at the entire hydrologic cycle.

There are three sources of water: the ground, the surface, and the atmosphere.

Ground WaterGround water supplies change more slowly. Though water can be pumped from the ground fairly quickly, recharge can be slow. Ground water recharge is typically measured in years.

Surface WaterBy far the most water exists on the surface – in the form of lakes, rivers, and oceans. In some locations significant amounts of water are also found in aquifers. Surface water is most accessible, and it can be easily diverted or pumped from lakes and rivers. Surface water recharge is also more rapid. A wet season can produce runoff that swells streams and reservoirs within months.

Atmospheric WaterAtmospheric water includes water vapor (humidity) and clouds. It’s the most variable source of water. Because the atmosphere’s capacity to hold water is directly linked to air temperature, more atmospheric water is found in temperate and equatorial regions than at the poles. The only way to access this water is through precipitation. What doesn’t fall to the ground moves on by – wherever the wind takes it. The availability of atmospheric water changes quickly – often as fast as the wind changes direction.

When we look at how cloud seeding affects the atmospheric water supply, we have to consider the following:

How much of the total water condenses to forms clouds?

How much of that condensed water forms precipitation and becomes surface water?

How much will cloud seeding affect the amount of water that falls to earth?

Water Condensation and Cloud FormationClouds form when moist air is sufficiently cooled. This usually happens when air rises, either as a result of surface heating (spring and summer cumulus clouds and showers), or forced flow over barriers, such as mountains. When air rises to form clouds, it must sink somewhere else, which usually happens somewhere nearby.

When clouds do form, the adjacent cloud-free air is not free of moisture. It just didn’t get cool enough to condense the moisture that exists. In general, a third of the moisture in the atmosphere might actually condense to form clouds. So, we can assume that 33% of the moisture has formed clouds while the other 67% is still present, but remains uncondensed.

Development of PrecipitationClouds never convert all of their condensed water into precipitation. Less than one-third (33%) of a cloud’s condensed moisture falls to earth as rain, snow, hail or drizzle.

If we have very efficient natural clouds that deliver a full 33% of their condensed water, then the total fraction of water vapor removed by natural precipitation would be a third of a third! That is about 11% of the existing atmospheric water.

The Effects of Cloud SeedingIf a cloud seeding project seeds clouds at the right time, from the right places and at the right time, it is possible to achieve increases in precipitation from 10-15%. If nature is being very efficient and it converts 11% of the total atmospheric water vapor to precipitation, then an increase from cloud seeding of 15% (0.15) of the 11% (0.11), or (0.11 x 0.15 = 0.017) just 1.7% of the total atmospheric water.

After taking other factors into consideration, the true impact of a cloud seeding operation is smaller yet.

Seeding is conducted only a small fraction of the time.Seeding does not occur around the clock.

Target areas for seeding projects are limited.The areas upwind and crosswind of a location are not affected.

In winter projects targeting mountain clouds, much of the flow that occurs is actually around the mountain, not over it.Air at the top of the mountain would not be seeded. Imagine a boulder in a shallow river; some water flows over the top, but much more flows around it. In such cases the fraction of air moisture that condenses to form clouds reduced even further.

It’s also important to realize that the atmosphere itself recharges (re-humidifies) by simply passing over water, snow, and vegetation. The water vapor that supplies storms that move west to east across the United States, typically comes from the south.

Studies of wintertime water budgets in the mountainous western United States, suggest that cloud seeding might reduce the humidity by up to 1% for a short period of time. Other studies that examined precipitation downwind of cloud seeding projects could find no impact at all.

Environmental impact from cloud seeding operations can be broken into two categories:

The effects of the chemical seeding agents used for the programs

The impacts of precipitation changes

Environmental Impact from Cloud Seeding AgentsThe active ingredient in cold cloud seeding agents is silver iodide, or AgI. Unlike ionic silver, the silver iodide compound is very stable, essentially inert. Numerous studies have been conducted that clearly demonstrate its safety. Even projects operated for decades have experienced no adverse environmental impacts because of silver iodide. Additional information about the safety of silver iodide, is found in many studies and reports. The active ingredient in warm cloud seeding is simple salt, either sodium chloride (NaCl, table salt), or calcium chloride (CaCl). Both are found abundantly in the environment; sea spray releases millions of tons of NaCl into the atmosphere annually.

Environmental Impact of Precipitation ChangesThe effects of cloud seeding on precipitation are typically on the order of 10%, while seasonal natural variability may be as much as a factor of two. When viewed in this context, the seasonal effects resulting from increased precipitation are negligible.

In the United States, most states have laws in place that require permits for all cloud seeding activities, and that those persons conducting the projects be qualified to do so. In addition, site-specific permits and/or environmental evaluations may sometimes be required. U.S. Federal law requires that all cloud seeding operations be reported annually to the National Oceanic and Atmospheric Administration (NOAA).

Depending upon the location and nature of the program, we may be able to mobilize an effective program within a few weeks of a signed contract. The fastest response generally includes the use of facilities owned by Weather Modification, Inc., including aircraft, radars and ground-based seeding equipment. If a client wants to use aircraft they already own, additional time is required to make required aircraft modifications and obtain needed certifications. Because of our extensive experience and resources, Weather Modification, Inc., does this faster than anyone.

Weather Modification, Inc. has the corporate infrastructure, experience, personnel – and all of the most modern equipment you might need for your project. Our Fargo, ND facility is the world’s largest, and the best-equipped of its kind. Having the right equipment at hand means we can mobilize faster, and get your program into operation quickly and effectively.

We operate a fleet of reliable twin-engine aircraft suitable for virtually every project, from warm season cloud base seeding operations, to wintertime operations in known icing conditions, to on-top convective storm treatment and/or atmospheric assessment and evaluation. We can even handle your high-altitude (up to 43,000 ft) research needs.

If you wish to use your own aircraft, we can quickly install all requisite seeding equipment and instrumentation for you.

The cost of a project depends upon the target area size, season and topography, types of seeding to be conducted, needed equipment and personnel, and the length of the desired project period. It is usually better to consider benefit-to-cost ratios. If you think you might be interested, contact us and we’ll be happy to assess your needs and offer an estimate.

You can contact us by telephone, mail, fax – or via our simple online form. Or if you prefer, e-mail us at info@weathermodification.com. Tell us where and when you need your project, what you hope to accomplish, and we’ll help you get started.